Short high-frequency antenna and feed system therefor

ABSTRACT

An antenna system is described wherein a short antenna of about 0.1 lambda or less is employed with a coaxial cable feed. The coaxial cable is directly connected to ground at but one end thereof and is coupled to ground at the other end via a capacitor. The capacitor is selected to provide good matching of a transceiver to the antenna with or without a coupler. In an alternate antenna system a loading network is employed which is mounted within the antenna to provide a stably performing antenna throughout adverse weather conditions such as rain, snow or ice.

United States Patent [72] Inventor John L. l-leins Massapequa Park, LongIsland, N.Y. [21] Appl. No 789,644 [22] Filed Dec. 11, 1968 [45]Patented June I, 1971 [73] Assignee Aero Systems, Inc.

Miami, Fla.

[54] SHORT HIGH-FREQUENCY ANTENNA AND FEED SYSTEMTHEREFOR ISCIaimSDraWingFigs. s2 U.S.Cl. 343/750, 343/830, 343/831, 343/850 511 1111.01.H0lg9/30 so FieldofSearch 343/s2s- [56] References Cited UNITED STATESPATENTS 2,615,131 10/1952 Lindenblad 343/831X 2,913,722 11/1959Brueckmann 343/830X 3,358,286 12/1967 Heins 343/750 PrimaryExaminer-Herman Karl Saalbach Assistant Examiner-Marvin NussbaumAttorney-Hopgood and Calimafde /4 RAD/0 Mull/981 or J- J CWPZER 4 FIG.4

? INVENTOR.

JOHN z. l/f/NS PATENIED JUN 1 I97! FIGS FIGS

SHORT MIGlH-FREQUIEIWIY ANTENNA AND FEED SYSTEM THEREFOR this inventionrelates to an antenna system. More specifically, it relates to anantennasystem for aircraft communication generally in and below the highfrequency range.

In the communication field, antennas are employed which, for practicalconsiderations, must have a very small electrical length. For instance,a typical short length antenna usable in the high frequencycommunication band from 2 to 30 MHz, is described in my Pat. No. MHZ.Short antennas are needed when one considers the dimensions of anantenna of one or several wavelengths long in comparison with theaircraft or building which is to support such antenna. The wavelength at30 MHz. is of the order of 32%feet and the wavelength at 2 MHZ. is about500 feet. Since it is generally desirable for efficiency and impedancematching considerations to communicate with antennas whose length is ofthe order of a quarter, a half or a full wavelength of the frequency atwhich communication is sought, it is clear that such antennas areimpractical for aircraft applications. The aircraft itself is generallynot longer than the wavelength at the low end of the high frequencycommunication band, i.e. 2 MHz. and even if future aircraft were to havesuch dimensions, it is impractical to consider employing an externallymounted antenna of such length when one considers the high operatingspeeds of jet aircraft.

When one employs antennas such as those used in the 2 to 30 MHZ. band,the radiation resistance of the antenna decreases. Practically, thismeans that if one were to use, for instance, a 4-foot-long antenna, theeffective electrical length of the antenna at 30 MHz is approximately0.125 A and at 2 MHz the electric length equals approximately 0.008 A.The corresponding decreases of the effective radiation resistance of theantenna becomes quite noticeable so that the antenna is not easilymatched and the transmitter power is not efficiently radiated by theantenna. In addition, very short antennas tend to become very-reactive,requiring a variable loading capacitor and an antenna coupler. Asdescribed in my above-mentioned patent, an antenna coupler generally andpreferably is placed adjacent the short antenna to tune out the variousreactances throughout the range of desired operating frequencies. Inthis manner a substantially resistive impedance is presented to theradio transmitter or receiver connected to the coupler at the other endthereof.

In practice, however, it quite frequently occurs that the an tennacoupler cannot be physically located near the antenna. One may envisagesuch location problems which it is realized that an antenna couplerwhich includes a variable inductance in series with the line and aparallel variable capacitor weighs approximately 12 lbs. and occupies aspace of about half of a cubic foot. In aircraft, such spacialrequirement cannot easily be provided directly adjacent the antenna.Since an antenna frequently is located at aircraft extremities, theadjacent location of a heavy coupler may appreciably affect the locationof the crafts center of gravity. For a tail-mounted antenna, the roomavailable can be too small for the coupler so that an intermediatenetwork such as a coaxial cable must be employed. To one skilled in theart, it can be appreciated that the insertion of a coaxial cable ofconstant characteristic impedance between a coupler and a highlyreactive antenna effectively transforms the impedance of the antenna toa high virtually purely reactive component. This means that when thecoupler tunes out the reactive components, the net resistive componentpresented to the transmitter or receiver tends to be exceedingly low.

Another factor that has been generally detrimental to a remotely locatedcoupler is that the coaxial cable loss will vary throughout the highfrequency spectrum from 2 to 30 MHz. with peaks and valleys separatedfrom one another by as much as 30 to 40 db. The precise frequencylocation of such valleys and peaks will of course very with differentlengths of coaxial cable. Since the 30 to 40 db. variation figure justquoted includes optimum adjustment of the antenna load capacitor and thecoupler, it can be seen that any improvement that would reduce suchlarge variations would greatly and reliably improve communication in thehigh'frequency passband.

It is therefore an object of this invention to provide an antenna systemwhich presents a low cable loss with a short antenna.

It is a further object of this invention to provide an antenna systemwhich includes a coupler that may be remotely located from a short,highly reactive antenna.

It is still further an object of this invention to provide an antennasystem for an aircraft or alike small size vehicle wherein the antennaemployed has a small effective electrical length yet performs with greatefficiency in the transmission of systems and with little loss ofreceived signal.

It is yet another object of this invention to provide an antenna systemfor the high frequency band or below, wherein a short antenna isemployed of an effective electrical length generally less than about 0.1of the operating wavelength.

The above-mentioned and other features and objects of this invention andthe manner of attaining them will become more apparent and the inventionitself will best be understood by reference to the following descriptionof embodiments of the invention taken in conjunction with theaccompanying drawings, the description of which follows.

FIG. 1 is a diagrammatic view of an antenna system as employed with anaircraft in accordance with my invention;

FIG. 2 is a generalized diagrammatic view of the embodiment of FIG. 1;

FIG. 3 is a diagrammatic view of another embodiment of the invention;

FIG. 4 is a diagrammatic view of still another embodiment in accordancewith the invention; and

FIG. 5 is a diagrammatic view of a rearrangement of the embodiment ofFIG. 3.

Briefly stated, my invention contemplates a short antenna of aneffective electrical length generally less than about onetenth of theoperating wavelength and wherein an antenna feed system is employed ofthe coaxial cable type. The coaxial cable outer conductor is selectivelyconnected to ground at a first location by a direct connection and at asecond location with a capacitor selected to 'provide a low loss antennasystem.

In F IG. 1 is shown the rear fuselage of a plane 10 to which is mountedat the rear end thereof an antenna I2 which protrudes therefrom and isgenerally called a tail probe. The antenna is driven by a transmitter 14via an antenna coupler 16 which is remotely located from the antenna andinterconnected thereto by a coaxial cable 18. The antenna 12 is madegenerally in accordance with my Pat. No. 3,358,286 and com prises ahollow tube 20 of a length La which is generally less than aboutone-tenth of the operating wavelength. The hollow tube 20 has an innerconductor 22 which electrically and mechanically connects to the tube atthe remote end 24 thereof. The other end of the inner conductor 22 isconnected to ground via a capacitive loading network such as capacitor26. The antenna tube 20 is insulatively mounted to the fuselage as isillustrated by the circumferential gap 28 and is fed by a cable systemcomprising the coaxial cable 18 from the coupler. The coaxial cable 18is of the standard SO-ohm or 75- ohm type and includes an innerconductor 30 and an outer conductor 32. The outer conductor generally isgrounded and in this instance is left floating at the antenna end whereit is connected or coupled to ground through a capacitor 34, hereinafterreferred to as the cable outer conductor grounding capacitor. The otherend of the coaxial cable 18 near the coupler I6 is grounded by a directconnection to ground. For il- Iustration purposes, the length of thecoaxial cable is indicated at Le and the spacing from the end of theouter conductor 32 to the fuselage end of the antenna is indicated as S.

The loading capacitor 26 generally is selected to provide adequatetuning capability of the antenna. The cable grounding capacitor 34,however, is selected commensurate with the length of the coaxial cablein order to provide the proper im-' pedance transformation needed topresent the proper electrical resistance to the transmitter or receiver.Although it would be considered convenient to provide a generalizedformula for determining the size of the loading capacitor and the cablegrounding capacitor, as well as the length of the coaxial cable, inpractice such installations do not lend themselves to rigorousmathematical analysis. Part of the reason resides in the fact thataircraft configurations vary as widely as can be imagined and theirsizes affect the performance of an antenna.

Typically, in one aircraft the tail probe will be the most efficient andeffective antenna for the high frequency communication band, whereas foranother craft the bellymounted probe will produce better results. Onecan appreciate such variations when the wavelength of the communicationfrequency is taken into account. Typically, an aircraft total wing spanlength may be of the order of the wavelength employed. Thus, theaircraft is likely to affect the antenna pattern as well as theradiation resistance, and the reactance of the antenna. Sincepredictability is not a practical method for determining the size of thecapacitors, other methods are employed. It is preferred that thecapacitors are selected as follows.

First, the antenna is mounted on the craft at a suitable location andtests are performed to determine first the optimum loading capacitor andcable grounding capacitor in conjunction with the coupler tuning effect.One may perform such tests by either transmitting a known signal ofknown intensity level through the coupler to the antenna and monitoringthe output as a function of the various capacitor values, or bymeasuring the input impedance of the antenna and coupler at thetransmitter. By recording the capacitive values for the optimum orlargest signal strength (or highest radiation resistance) over variousfrequencies throughout the band of interest, the 2 to 30 MHz. band, aset of data is obtained to select the capacitor 34 value. Such procedurefor determining the various elements is well known in the art andfurther detailed description is not necessary. Suffice it to say here,however, that even where some compromises must be made, the improvementobtained by the placement of a cable ground capacitor 34 at the antennaend is so significant that the 30 to 40 db. variations referred to inthe introduction are reduced to a ripple which has a peak value of about4 db. Such small 4 db. fluctuation is considered barely noticeable sincethe aircraft orientation usually introduces variations in the receivedsignal well in excess of such fluctuation.

In FIG. 2 I show a generalized version of the embodiment of FIG. 1wherein a radio transceiver 14 is employed supplying and receiving asignal through the coupler 16 which is remotely located from the tubularantenna and connected thereto via a coaxial cable 18. The cable adjacentits antenna end is provided with a cable ground capacitor 34. Note thatthe load capacitor 26 is indicated as variable but the cable groundcapacitor is indicated as a fixed value. For some aircraftconfigurations it is entirely possible that for optimum performancecapability, both the load and the cable ground capacitor are variable.This may be easily accomplished by applying a pair of motors (not shown)in operative relationship with the cable ground and loading capacitors34, 26. The motors are controlled by the coupler 16 to establish thedesired capacitive values.

In some installations such as of the fixed frequency type, a coupler fortuning of the short antenna is not needed and a fixed tuning capacitormay be employed. FIG. 3 is illustrative of such an embodiment wherein aradio transceiver is employed that is connected to the antenna via acoaxial cable 18 which, near the antenna end thereof, has its outerconductor connected to ground through capacitor 34. At the radiotransceiver end, the outer conductor 32 is directly coupled to ground.By again selectively choosing the capacitor both for loading and cablegrounding, a highly efficient transmitting system is obtained.

FIG. 4 is illustrative of an antenna system which is especially usefulfor low frequency (LF; 200 to 415 kHz.) transmissions such as in the 200kHz. range. Again I employ a tubular antenna but within the antenna Imount a loading network 36 so that it may emerge from the bottom end ofthe antenna 12 at ground potential. In the particular embodiment of FIG.4, the loading network comprises a coil 38 and a capacitor 26' tuned toprovide the desired loading at the 200 to 415 kHz. frequency. Theantenna is either fed by a coaxial cable 18 from a radio transceiverdevice 14 or from a coupler 16 closely mounted thereto. The cable 18 atthe antenna end has its outer conductor connected to ground via coaxialcable grounding capacitor 34.

A significant advantage of the embodiment of FIG. 4 resides in the factthat the center conductor after passing through the loading network 36emerges from the antenna at electrically ground potential. Thus, theaccummulation of dirt, snow or rain does not have a strong effect on theperformance. Typically, the length of the antenna of FIG. 4 may beapproximately 60 feet which, at the wavelength of 1500 meters, providesan effective electrical length of approximately 0.0l225 of the operatingwavelength. Yet, I have found that by employing an antenna system suchas shown in FIG. 4, a relatively small ground plane may be used withouta significant variation in the power radiated from the transmitter dueto environmental effects.

In FIG. 5 I show a similar antenna system to that shown in FIG. 3, butthe coaxial cable '18 in FIG. 5 has its antenna end directly coupled toground and its coupler or radio transceiver end connected to groundthrough a cable grounding capacitor 34. The embodiment of FIG. 5 wouldbe especially useful where both the cable ground capacitor 34 and theloading capacitor such as shown in FIG. 2 have to be variable. Thevariability of the cable grounding capacitor in the configura tion ofFIG. 5 may be simply accomplished by merely attaching a wafer to arotary switch that usually is employed within the coupler.

The hollow antenna used with this invention operates with very shortelectrical lengths. For instance, I have found that an antenna tubelength of about 14 inches operates satisfactorily over the frequencyrange from to 1600 kHz. The electrical length of the antenna thus may beas small as 0.0001 of the operating wavelength. Such short antenna hasparticular utility on an aircraft.

While the principles of the invention have been described in connectionwith specific apparatus, it is to be clearly understood that thisdescription is made only by way of example and not as a limitation tothe scope of the invention as set forth in the objects thereof and inthe accompanying claims.

I claim:

I. An antenna system comprising an antenna including a hollow conductingtube having an electrical wavelength of less than 0.1 of the operatingwavelength of the antenna, a coaxial cable feed connected to the antennaand having an inner and an outer conductor, said outer conductor beingconnected to ground at a preselected location thereof, a cable groundingcapacitor electrically coupling a predetermined location of the cableouter conductor to ground, said hollow conducting tube further includingan inner conductor electrically and mechanically connected to a tubeend, and a loading network coupling the other end of the tube innerconductor to ground.

2. The antenna system as recited in claim 1 wherein said capacitorconnects the outer conductor at its antenna end to ground.

3. The antenna system as recited in claim 1 where said capacitorconnects the outer conductor to ground at an end thereof away from theantenna.

4. The antenna system as recited in claim ll wherein said loadingnetwork is mounted within the tube and emerges from said tube atsubstantially electric ground level.

5. The device as recited in claim 4 wherein the loading networkcomprises an inductance and a capacitance in series connection with oneanother.

6. The device as recited in claim 1 wherein the electrical length of theantenna is of the order of between 0.0001 to 0.00l of the antennaoperating wavelength.

7. An antenna system comprising an antenna formed of a hollow conductingtube with an inner conductor-having an end electrically and mechanicallyconnected to the tube at one end thereof, said tube having a lengthgenerally less than about 0.1 of its operating wavelength.

3 load capacitive reactance coupling the other end of the innerconductor to ground,

a coupler for coupling transmitting RF signals to the antenna andreceived RF signals from the antenna, said coupler being remotelylocated from said antenna,

a coaxial cable electrically interconnecting the remotely locatedcoupler to the antenna and having a center conductor enclosed by anouter conductor,

said outer conductor being directly connected to ground and a cableouter conductor grounding capacitor coupling a predetermined location ofthe outer conductor to ground. 8. The device 'as recited in claim 7wherein said cable grounding capacitor is selected commensurate with thelength of the coaxial cable'to provide optimum transmission capabilityof the antenna over its operating frequency.

9. The device as recited in claim 7 wherein both said load capacitivereactance and said cable capacitor are variable for optimum performanceof the antenna over its operating frequency.

10. The device as recited in claim 7 wherein said grounding capacitor isselected commensurate with the minimum optimum loss of signal in thecable over the operating frequency of the antenna.

11. The device as recited in claim 7 wherein said cable groundedcapacitor is selected to provide optimum performance of the antenna atthe low end of the operating frequency.

12. The device as recited in claim 7 wherein said cable groundedcapacitor connects the antenna end of the outer conductor to ground.

13. The device as recited in claim 7 wherein said cable groundedcapacitor connects the coupler end of the outer conductor to ground.

1. An antenna system comprising an antenna including a hollow conductingtube having an electrical wavelength of less than 0.1 of the operatingwavelength of the antenna, a coaxial cable feed connected to the antennaand having an inner and an outer conductor, said outer conductor beingconnected to ground at a preselected location thereof, a cable groundingcapacitor electrically coupling a predetermined location of the cableouter conductor to ground, said hollow conducting tube further includingan inner conductor electrically and mechanically connected to a tubeend, and a loading network coupling the other end of the tube innerconductor to ground.
 2. The antenna system as recited in claim 1 whereinsaid capacitor connects the outer conductor at its antenna end toground.
 3. The antenna system as recited in claim 1 where said capacitorconnects the outer conductor to ground at an end thereof away from theantenna.
 4. The antenna system as recited in claim 1 wherein saidloading network is mounted within the tube and emerges from said tube atsubstantially electric ground level.
 5. The device as recited in claim 4wherein the loading network comprises an inductance and a capacitance inseries connection with one another.
 6. The device as recited in claim 1wherein the electrical length of the antenna is of the order of between0.0001 to 0.001 of the antenna operating wavelength.
 7. An antennasystem comprising an antenna formed of a hollow conducting tube with aninner conductor having an end electrically and mechanically connected tothe tube at one end thereof, said tube having a length generally lessthan about 0.1 of its operating wavelength. a load capacitive reactancecoupling the other end of the inner conductor to ground, a coupler forcoupling transmitting RF signals to the antenna and received RF signalsfrom the antenna, said coupler being remotely located from said antenna,a coaxial cable electrically interconnecting the remotely locatedcoupler to the antenna and having a center conductor enclosed by anouter conductor, said outer conductor being directly connected to groundand a cable outer conductor grounding capacitor coupling a predeterminedlocation of the outer conductor to ground.
 8. The device as recited inclaim 7 wherein said cable grounding capacitor is selected commensuratewith the length of the coaxial cable to provide optimum transmissioncapability of the antenna over its operating frequency.
 9. The device asrecited in claim 7 wherein both said load capacitive reactance and saidcable capacitor are variable for optimum performance of the antenna overits operating frequency.
 10. The device as recited in claim 7 whereinsaid grounding capacitor is selected commensurate with the minimumoptimum loss of signal in the cable over the operating frequency of theantenna.
 11. The device as recited in claim 7 wherein said cablegrounded capacitor is selected to provide optimum performance of theantenna at the low end of the operating frequency.
 12. The device asrecited in claim 7 wherein said cable grounded capacitor connects theantenna end of the outer conductor to ground.
 13. The device as recitedin claim 7 wherein said cable grounded capacitor connects the couplerend of the outer conductor to ground.